1. Field of the Invention
This invention relates generally to a method of forming substantially long hollow concrete slabs having one or more continuous holes extending therethrough along the length thereof, and more particularly to a core for use in casting such slabs for forming such continuous holes.
2. Prior Art
Hollow concrete slabs have been extensively used in the construction of various kinds of structures including buildings, railway station platforms, express ways and the like because they are superior to the conventional non-hollow concrete slabs in structural strength, weight, production cost and so on. Because of these advantages, there has now been a great demand for such hollow concrete slabs. However, the casting of the hollow concrete slabs is far more complicated than that of the non-hollow concrete slabs and therefore requires advanced techniques. For this reason, such hollow concrete slabs have not been produced by the manufacturers in an amount enough to fully meet the increasing demand. The primary difficulty encountered in casting a long continuous hollow concrete slab has been the problem of providing an efficient core means for forming a continuous hole extending longitudinally through the concrete slab. None of the heretofore-proposed core means have been found entirely satisfactory. Typical examples of such conventional core means are as follows:
One such known core comprises a tubular body made, for example, of a tinplate, the tubular body being embedded in the cast concrete slab so that the bore of the tubular body defines a continuous hole extending longitudinally through the concrete slab. Thus, such core is designed to be permanently retained in the cast slab, and it is rather difficult to select the optimum material of which the core is to be made. Another disadvantage is that this procedure increases the overall manufacturing cost becaused of such added material. A further disadvantage is that where a long hollow slab is to be cast, it is quite difficult to place such a long core in position in a mold or casting bed. Thus, this conventional core has been found not entirely suited for use in casting a long hollow concrete slab.
Another conventional method of casting a long hollow concrete slab is known in which a hopper for feeding concrete into a horizontally-disposed mold is mounted above the mold for movement therealong. The hopper is provided with a core element which has a vertical portion extending downwardly from the hopper at its front and a horizontal portion extending rearwardly from the vertical portion beyond the opening of the hopper through which concrete to be cast is fed into the mold, the horizontal portion being disposed in the mold and serving to form a hole through the cast concrete. The hopper is advanced in a casting direction to pour concrete into the mold so that the horizontal portion of the core element is simultaneously moved to form a hole through the cast concrete. With this method, the horizontal portion of the core element is continuously moved through the cast concrete to form a continuous hole immediately after the concrete is cast into the mold. Therefore, the thus formed hole is susceptible to deformation or fracture. If the slump is kept small in order to overcome this difficulty, the bonding between the cast concrete and the associated longitudinal and lateral reinforcing bars would be adversely affected so that cracks may occur in the resultant concrete slab. Thus, this method has failed to provide a long hollow concrete slab of sufficient strength and durability.
A further conventional method of casting a hollow concrete slab employs a core of granular material to provide a continuous hole. In this method, a pair of first and second hoppers are mounted above a horizontal long casting bed for movement in unison therealong. The first hopper for extruding the granular material includes a core-forming element having opposite open ends, and feed means interconnecting the hopper and the core-forming element, the core-forming element being arranged to be disposed in the casting bed and beneath the second hopper. During the casting operation, the first hopper extrudes the granular material to the core-forming element through the feed means so that the granular material is compacted by the core-forming element to provide part of the core to be formed. The granular material extruded into the core-forming element is immediately compacted, and since the first and second hoppers are continuously moved in the casting direction, the core-forming element moves away from the thus compacted granular material immediately after it has been filled in the core-forming element. The second hopper is located rearwardly of the first hopper and pours concrete over the core-forming element to form a cast concrete slab. After the concrete slab is cured, the granular material embedded in the concrete slab and serving as the core is removed from the slab to provide a continuous hole extending longitudinally therethrough. With this method, however, difficulty has been encountered in properly compacting the granular material acting as the core. A further problem is that the so formed core can not be easily removed from the cast concrete slab. This difficulty arises from the fact that the core element naturally has a coarse surface because it is composed of the granular material. The coarse surface serves to enhance the binding of the cast concrete to the core. In addition, the coarse surface detracts from the appearance of the finished product and diminishes its quality. This is undesirable from a commercial point of view. Further, pressure air and the like are employed to remove the core of the granular material from the cast concrete slab. This procedure gives rise to substantial noises and dust. This is undesirable from an anti-pollutant point of view. For these reasons, this method has been found not wholly satisfactory.